1. Field of the Invention
This invention relates to seed coating techniques more particularly of coating seeds of plant species which enter into a symbiotic relationship with micro-organisms or else are beneficially effected by micro-organisms.
2. Prior Art
The desirability of inoculating legume seeds with an appropriate strain of rhizobia for the purpose of promoting plant growth is now well established. In fact in some countries such as New Zealand it is considered desirable for certain legume seeds to be inoculated with an effective strain of rhizobia bacteria before sowing. The primary effect of rhizobia bacteria is in the fixation of atmospheric nitrogen into a useable form of nitrogen for the plant. The rhizobia bacteria form nodules on the plant roots and are sustained by the plant and in turn provide nitrogen for the plant as mentioned above. Other micro-organisms do not enter into such a close symbiotic relationship with a plant but nevertheless when in close proximity to a plant can stimulate plant growth. Micro-organisms such as mycorrhizal fungi assist plant roots to absorb nutrients from the soil. Other plant growth assisting micro-organisms include but are not limited to Azotobacter, Bacillus spp and much research is continuing with other forms of such micro-organisms.
The effectiveness of rhizobia is now well documented as is the effectiveness of certain brands of commercially inoculated and coated seed. With other micro-organisms the effectiveness is not quite so marked but nevertheless this invention is considered as being applicable to the use of such other micro-organisms as well.
It has been found over a number of years, and there are many reports in the literature on such observations, that seeds possess certain substances which are toxic to micro-organisms. This effect has been particularly noted with the more actively researched legume seeds and the corresponding rhizobia bacteria. Certain researchers have characterised some of the toxins and have been able to show that such toxins possess anti-biotic activity in vitro against species of rhizobia. However the precise nature of all of the toxins in legume seeds is still the subject of some discussion in the literature.
To attempt to reduce the effect of toxins on micro-organisms such as rhizobia, a number of materials have been proposed to attempt to absorb or deactivate the toxins and thus prolong the life of the bacteria when present on the seed. Previously tried materials include charcoal, skim milk, casein, gelatin, glycerol, sucrose/yeast, sodium and other salts of casein and an insoluble polyvinyl-polypyrrolidone. Caseinate salts such as the sodium salt have shown useful improvement in rhizobia survival when used as an adhesive to bind coating materials including inoculant to the seed. The literature also reports tests on rhizobia survival in laboratory trials using an insoluble polyvinyl-polypyrrolidone sold under the trademark Polyclar AT by GAF Corporation (C.N. Hale Proc. N.Z. Grasslands Association Nov. 2-4, 1976 and Hale et al N.Z.J. Ag.Res. 20: 09-77).
Polyclar AT is defined by the manufacturers as polyvinyl-polypyrolidone and is an insoluble high molecular weight polymer. Its insoluble nature is no doubt in part created by the fact that it is cross linked. Hale reported the use of Polyclar AT, to inhibit the antibiotic activity of seed diffusates against isolates of Rhizobium species on agar cultures. The correlation between such in vitro trials and the effectiveness of such a material in assisting the survival of Rhizobium species on seeds is questionable. To be of any commercial use, the "detoxicant" must increase the survival of the rhizobia on the seed to a substantial extent to justify the additional costs involved in incorporating the material on the seed. Hale et al in the N.Z.J. Ag.Res. did measure the survival of rhizobia on seed in which Polyclar AT was mixed with white clover seed before inoculation and then inoculated and tested after 24 hours storage for survival of the bacteria. The amount of insoluble PVPP employed was 10% (w/w) of the seed. With such a treatment Hale et al reported that only 2.7% of the rhizobia bacteria survived after 24 hours in comparison with 0.3% with the control method.
The additional cost of the insoluble PVPP at such a concentration would be prohibitive and the apparent advantage achieved would be so minor as to be insignificant. In the experimental procedures of testing for numbers of rhizobia, the errors involved are so large that it is only after a statistically significant number of experiments can one say that 3% rhizobia survival is indeed different from 0.3% rhizobia survival.
The 1979 issue of the GAF Chemical Catalogue does state that the product identified as PVP/K-15 having an average molecular weight of about ten thousand does detoxify many poisons and irritants. However there is no mention that it can be used on seeds to improve the survival of micro-organisms which may have a beneficial effect on the seed.
As mentioned above the nature of the toxins that are exuded by seeds are still not totally defined. Moreover from the Hale et al article it is clear that the antibiotic effect of a seed diffusate in vitro can be stopped completely with Polyclar AT but when such a product is present in the seed coating the results which doubtfully indicate any significant improved survival of rhizobia bacteria.
Another article which does mention the use in general terms of polyvinyl-pyrrolidone is the Russian Chemical Review 43 (7) 1974 which reported that the quality of an applied coating of pesticides to a seed can be improved by introducing a polymeric film forming agent into the composition. One example is given of incorporating up to 15% of polyvinyl-pyrrolidone in the coating. The article does not clearly state whether the PVP being referred to is of the soluble or insoluble form and more importantly no mention is made that they are concerned with legume seeds or the problem of survival of rhizobia bacteria or any other micro-organisms on or used in conjunction with seeds.
Thus there has been a problem in the seed industry of being able to market a seed pre-inoculated with rhizobia bacteria to allow for inherent delays in marketing and freight and still ensure that sufficient rhizobia survive to perform their necessary function. In situ inoculation, namely where the farmer himself applies a culture of rhizobia bacteria to legume seeds immediately before sowing, has still therefore been widely practised to ensure that sufficient rhizobia bacteria are present at time of sowing.
Despite the fact that knowledge has existed in the art of the toxicity of certain materials in seeds to the symbiotic bacteria and attempts at reducing the toxic effects by using certain additives has been actively researched, the degree of improvements reported in the literature has been small.